Per Wikipedia the RAMAC 305 stored 5 million 8-bit characters[0]. Assuming they're using all 100 surfaces of the 50 disks in the unit that only comes out to 400,000 bits per surface. At the size of those platters the magnetic domains that encoded the bits must have been positively huge. There are products that could be used to visualize magnetic domains on tape[1]. The RAMAC platters seem like they'd be large enough that you could read them optically with one of these visualization tools.

[0] https://en.wikipedia.org/wiki/IBM_305_RAMAC

[1] https://www.tapeheads.net/threads/visualizing-magnetic-signa...